Link Whisper is a legitimate tool widely used for internal link building on WordPress sites. It utilizes Natural Language Processing (NLP) technology to analyze content and suggest internal links, making the process more efficient and less time-consuming. Users have found it to be user-friendly, affordable, and effective in improving their site's SEO. The tool has received positive reviews for its ability to save time and effort, although it may not be perfect in its link suggestions. Overall, it is considered a valuable investment for those looking to enhance their internal linking strategy.